Recently, digital video movie cameras (DVC) and digital still cameras (DSC) are beginning to prevail. The digital multi-pixel processing has made available DSCs of several hundred pixels. DVCs equipped with an image sensing element which is formed by more than 1,000,000 pixels and has a function of photographing even a still image are commercially available. Digital processing enables various corrections which are difficult in the prior art. For example, corrections such as scratch and noise corrections of an image sensing element can be generally done.
Digitization and multi-pixel image sensing elements give a still image photographing function to DVCs and a movie function to DSCs; the functions of DVCs and DSCs are coming close to each other. Recent cell phones are equipped with even a still image/movie image capture function, and these image input means have greatly been developed.
In this situation, some problems which were solved before have become serious again. One of these problems is smear. Smear has existed since solid-state image sensing elements became available. As is well known, smear occurs when incoming light during signal charge transfer serves as a false signal to generate a stripe, degrading the image quality. When the smear amount is large, even a dark portion vertically adjacent to a portion having high brightness (e.g., a general landscape having a sky image at an upper portion and a mountain or building at a lower portion) flares without any strong light source, and may even become reddish, degrading the image quality. This is because light incident upon transferring a signal from a high-brightness portion leaks in transferring a signal from a dark portion, and a false signal is added to the signal from the dark portion.
As a measure against smear, the history of the sensor structure has shifted from frame transfer (FT) CCDs to interline transfer (IT) CCDs. The IT-CCD is resistant to smear, and thus has become the mainstream of image sensing elements. In the FT-CCD, the pixel has both the signal storage function and transfer function. To the contrary, IT-CCDs have the functional structures of a photodiode (PD) having only the signal storage function and a vertical transfer CCD (V-CCD) having only the transfer function. Regardless of these advantages of IT-CCDs, IT-CCDs and FT-CCDs have equally been adopted at the beginning because IT-CCDs also suffer smear due to the mixing of light incident from the PD in the V-CCD. To reduce smear, the pixel structure of the sensor has constantly been improved to reduce leakage of light from the PD to the V-CCD.
However, smear which has been suppressed to a practically negligible level by continuous improvements is on the rise recently. Smear stands out particularly in EVF moving images and movie images obtained by multi-pixel DSCs. The main factor of this phenomenon is attributed to the fact that the protection against leakage of light from the PD region to the V-CCD region degrades due to a small cell size of the sensor along with the multi-pixel structure, and in the use of the DSC, pixel subtraction is executed for EVF moving images and movie images to increase the incident light quantity during the read time much more than that in the DVC sensor or the like. As for cell downsizing, even DVC products equipped with the still image capturing function require image sensing elements having more than 1,000,000 pixels. The pixel must be downsized, similar to the DSC, the smear protection degrades, and needs for a smear measure rise.
Under the present circumstances (improvements of the correction ability against various drawbacks caused by an increase in smear in the image sensing element and digitization), demands have arisen for realizing conventionally known smear correction. For example, Japanese Patent Laid-Open No. 2001-24943 proposes conventionally known smear correction and a measure against its problem.
In conventional smear correction, the V-CCD is operated by transfer stages larger in number than the vertical PD, and smear correction of each signal line is done using only information (smear line data) on smear superposed on a dummy line which does not output any PD signal. In this case, the number of transfer stages is set larger by several stages so as to keep several dummy lines. A plurality of dummy lines are added and divided by the number of lines to average the data. This reduces noise contained in smear line data. However, the read time is prolonged by the number of dummy lines, and the number of dummy lines cannot be increased so much. Dummy lines may be formed by shielding the upper PD portion of the image sensing element from light, instead of increasing the number of transfer stages.
In the following description, the type of smear correction is not classified by the dummy line formation method. Smear line data or a smear line signal means smear correction data having 1-line information by averaging.
Conventional smear correction will be summed up and described. The smear amount in columns uniformly appear in all lines, and an output of only a smear component is obtained from a dummy line which does not receive any light. The obtained smear line information is used in correction to obtain an image free from any smear by subtracting smear data of the same column from signal data of each signal line. This correction is subtraction:Sout(i,j)=Sccd(i,j)−Sm(i)  (1)                Sout: data after smear correction        Sccd: data before smear correction        Sm: smear data        i: horizontal address        j: vertical address        
However, this correction method decreases saturation by the smear subtraction amount. For example, let AD be the CCD output for a 10-bit AD. At this time, when the smear data value is 50, the signal is saturated at 1024−50=974.
This state is shown in FIG. 7. The solid line in FIG. 7 represents the input/output relationship without any smear, the dotted line represents the input/output relationship of a column having smear, and the chain line represents the input/output relationship after smear correction. The smear amount and a decrease in dynamic range upon correction are equal to each other.
This problem has also been pointed out in Japanese Patent Laid-Open No. 2001-24943 described above. This reference states that a portion having high saturation brightness becomes gray in a line having a large smear correction amount, resulting in an unnatural image. This phenomenon actually occurs. As a measure, this proposal describes a correction method:Sout(i,j)=(Sccd(i,j)−Sm(i))×Sat/(Sat−Sm(i))                Sat: saturation value        
When the smear amount increases, an offset is added. If the smear amount increases further more, no correction is performed. As a result, correction becomes linear without any decrease in saturation.
In this correction, the gain changes between columns, and a stripe is generated in an image, resulting in a low-quality image. In signal processing after smear correction, gamma correction is done, and especially the gain increases at low brightness. This further enhances the stripe at the dark portion of the image.
This proposal provides a method of changing the gain in accordance with the smear level and a method of inhibiting any correction for a larger smear amount. However, correction is done on the basis of information of each column, further increases the difference between columns, and presents a new cause for image quality degradation.
To prevent such new image quality degradation, clipping at the maximum smear value which is not preferable in the embodiment of this proposal is an effective means. However, direct use of this means causes a problem that “the image sensing range narrows to make the entire image dark owing to removal of a smear spot”, which is pointed out in the proposal.